This invention relates to the shaping of heat-softened glass sheets or the like by pressing the sheets between two rigid pressing members of complementary curvature. This technique, known as press bending, lends itself to relatively inexpensive mass production of bent glass sheets within precise tolerances. More particularly, the invention relates to press bending glass sheets so as to produce bends of very sharp curvature known as "V" bends.
Press bending in general is well known in the art, as exemplified by U.S. Pat. No. 3,367,764 to S. L. Seymour. Prior art press bending processes, however, have been limited as to the sharpness of curvature which may be imparted to the glass, a minimum radius on the order of about 9 inches (23 centimeters) or more being typical of commercial press bending installations. It would be desirable to produce "V" bends having radii less than 8 inches (20 centimeters), preferably on the order of 4 inches (10 centimeters) or less.
The production of sharp "V" bends has heretofore been most commonly carried out by gravity sag bending processes, wherein glass sheets are placed on contoured outline molds in a generally horizontal orientation and heated to the softening point of the glass so that the glass sags to conform to the shape of the mold. In order to cause such a bend to be concentrated along a so-called "hot line" to form a "V" bend, localized heat is applied along that line by electric resistance heating through a wire or ribbon held close to or in contact with the glass or through an electroconductive substance applied to the glass. The use of a heated wire or ribbon is disclosed in U.S. Pat. No. 3,795,501 to R. A. Jansson and T. J. Reese; U.S. Pat. No. 3,795,570 to R. A. Jansson and D. L. Thomas; U.S. Pat. No. 3,847,586 to T. J. Reese, G. R. Claassen, and M. W. Tobin; U.S. Pat. No. 2,111,392 to J. H. Galey; U.S. Pat. No. 2,176,999 to R. A. Miller; U.S. Pat. No. 2,215,228 to J. G. Oliver; U.S. Pat. No. 3,248,195 to J. S. Golightly and H. E. McKelvey; and U.S. Pat. No. 2,999,338 to R. L. Richardson. The use of electroconductive stripes applied to the glass surfaces is disclosed in U.S. Pat. No. 3,762,903 to H. E. Hamilton; U.S. Pat. No. 3,762,904 to H. E. Hamilton, R. E. Bamford, and P. Pastorek; U.S. Pat. No. 3,879,184 to H. E. Hamilton and I. L. Soreghy; and U.S. Pat. No. 3,865,680 to T. J. Reese and H. S. Koontz.
Prior to this invention it has been proposed to apply localized heat to glass sheets in a press bending operation, but not in a manner compatible with forming "V" bends. One such proposal is disclosed in Canadian Pat. No. 799,907 to R. E. Richardson where the leading edge of each glass sheet is heated in the press bending station in order to equalize the temperature of the sheets from leading edge to trailing edge. The heat is provided by radiant heaters aimed at the general region of the sheet near the leading edge. U.S. Pat. No. 3,333,935 to C. E. Valchar and S. J. Mrozinski shows a press bending method wherein the lower portion of each glass sheet is given additional heat by means of a pair of gas burners in order to enable that portion of the sheet to be bent to a sharper curvature. Such an arrangement, however, does not produce the narrow, precisely located lines of heating which are required for making "V" bends. Another use of localized heat during press bending is disclosed in U.S. Pat. No. 3,960,535 to H. E. Hamilton and W. W. Oelke, which discloses the application of heat with gas burners to the portion of glass sheets around holes in the sheets during press bending. The object is to maintain equalized temperature conditions in the glass, and therefore has very little in common with the production of "V" bends. Canadian Pat. No. 900,720 shows a press bending mold with a plurality of elongated electrical heating elements extending along vertical lines in the mold, but the heating elements are beneath the surface of the mold since their purpose is to warm the entire mold face, not to impart localized heat to the glass.